Background illumination, otherwise referred to as backlighting, is commonly used in information display units, rubber keypads, membrane switches, liquid crystal displays, rigid panels and the like, to make them more discernible and to enhance their visibility. Some existing techniques utilize fiber optics for this purpose.
Typically, such prior backlighting devices utilize a plurality of optical conductors, each having a core surrounded by cladding, which are placed in intimate proximity to each other above an optically reflecting surface. Light is beamed into the optical conductors at one end using a simple light source, such as a lamp. The light beamed in is propagated in the core of the optical conductor by means of partial internal refraction.
In accordance with one prior technique, at a desired location on a portion of the optical conductors positioned behind the panel to be illuminated, abrasions are formed in the surface, typically by using a hot stamping machine. The stamping machine forms the abrasions by pressing against the optical conductors at the desired locations, in a random manner. As the light passes down the optical conductors, a portion of the light exits through each of the abrasions in the surface and illuminates the location directly above that region. Typically, the remaining light continues its travel along the optical conductors and terminates at a location remote from the abrasions, thus creating inefficient illumination in the abraded area.
U.S. Pat. No. 4,845,596 to Moussie discloses one such technique whereby an outer sheath of the optical conductor is removed locally and a portion of the light beamed through the optical conductors escapes therefrom at those points. This emerging light reflects off the reflective backing and illuminates the surface above.
In such prior devices, the light which exits through the abrasions in the surface provides backlighting of relatively low efficiency relative to the amount of light beamed in. Also, the intensity of light along the fiber optic cable diminishes as light is refracted through the abrasions along the fiber optic cable. Moreover, some light continues to the end. Thus, although such prior devices are known to serve their purpose, they have not proven to be satisfactory.
In accordance with yet another technique, optical conductors are held together by a thread which is tightly woven around the optical conductors, thereby creating corrugations, the angled sides of which exceed the acceptance angle or numerical aperture of the cladding and allow some of the light beamed through to escape. The amount of light which escapes can be limited by controlling the tightness of the weave. In applications not requiring diffusion, such optical conductors, in addition to providing relatively low intensity of light, are difficult to use behind key pads due to their thickness.
Most of the prior techniques involve manually placing staggered layers of optical conductors which has proved to be laborious, inefficient and economically unfeasible.
A need thus exists for an improved fiber optic backlighting panel and technique for making fiber optic panels for providing increased and uniform background illumination in a device relative to the amount of light beamed in.